Autoimmune (idiopathic) thrombocytopenic purpura (ITP) is a disease caused by autoantibodies directed against platelets, but the demonstration of specific antibodies has been difficult for a variety of reasons. In general, when the antibodies can be demonstrated, there is an inverse correlation with the platelet count in individual patients. We have set up an assay for specific platelet glycoproteins, to aid in the diagnosis, treatment, and monitoring of patients with ITP. We are using the tests particularly for the followup of patients before and after treatment in 2 studies: with NHLBI in the treatment setting of T-cell-depleted auto-stem cell transplantation in patients with severe ITP, and in an ITP treatment protocol for less severely affected patients using Daclizumab. Sixteen patients have been studied in the transplant protocol and 11 have been studied in the daclizumab protocol. The daclizumab protocol has had only one partial responder with a significant stable elevation in platelet count.